PTFM-12054 Fix dxpy coverage report

Summary:
Explicitly pass the COVERAGE_PROCESS_START env var to the
python process that runs the dxpy tests. This ensures coverage is
measured in subprocesses. Also updated the test selection option
(--testfile) to take an entire unittest file, as a stopgap until
the existing per-test method selection option (--tests) can be
tweaked to work again.

Test Plan: Tested locally and on Jenkins

Reviewers: psung

Reviewed By: psung

Subscribers: akislyuk

Differential Revision: https://phabricator.dnanexus.com/D75